Going inwards the ovule is made of: – Two integuments, the outer one being vascular. At the ovule apex, there is a tiny opening in integuments called micropyle. – The nucellus is a diploid parenchymatous tissue, surrounded by integuments and containing the embryo sac. If the nucellus is thin, the ovule is tenuinucellate; if it is thick, it is crassinucellate. – The embryo sac, the female gametophyte, is made of 8 haploids cells.
The pollen grain of pollen is generally made of only 2 haploid cells: – The tube cell, responsible for the elongation of the pollen tube, – The generative cell that will give 2 male gametes or sperm cells. The pollen grain is surrounded with 2 walls: – The exine, thick, ornamented or smooth, – And the intine, fine and doubling inside the exine. In certain places called apertures, exine is thinner and intine thicker. In the pollen of Monocots or Basal Dicots, there is normally only one aperture, circular or furrowed. Sometimes, this aperture stretches, being thinned by places it then gives several pores. In the pollen of true Dicots (Eudicots), there are normally 3 apertures. However, this number can vary and be multiplied.
Germination of pollen begins when the grain swells because of the absorption of the water at stigma surface. Turgescent vacuoles push the intine and the cytoplasm towards an aperture of the pollen grain of pollen: it is the beginning of the growth of the pollen tube. When the pollen tube gets longer, the 2 cells (generative cell and tube cell) and the cytoplasm are confined at its extremity, the generative cell being slightly behind the tube cell. Phenomena of cellular strain are limited at the end of the tube. The pollen tube crosses the style without ever penetrating into cells; cells of the conducting tissue act as rails for the pollen tube that finishes its growth at the level of the micropyle into the ovule.